Serial and parallel hybrid systems are well known and generally seek to better match engine performance characteristics, to the demands of operation, in order to improve efficiency with regard to fuel consumption and transmission of power.
It is known to provide a parallel hybrid system, especially for boats, in which an internal combustion is engine is connected to drive a propeller through a gearbox, and a motor/generator is connected to the gearbox input or output through a clutch. The propeller can be driven by the internal combustion engine when required. In addition the motor/generator can charge the battery when the engine is driving the propeller. The motor/generator (as powered by the battery) can drive the propeller when the engine is not working. Alternatively the propeller can be used to drive the motor/generator to charge a battery—e.g. if sailing.
Such known parallel hybrid systems fall into two types namely flywheel parallel hybrid systems in which a the engine output is connected via a clutch to a motor generator in series with and integral with a gearbox (see FIG. 1A) which drives a propeller shaft, or an external parallel hybrid in which an engine is connected to a gearbox which drives a propeller shaft through a clutch, and a motor generator is connected externally to the output of the gearbox between the gearbox and the clutch (see FIG. 1B). A problem with both these systems is that a failure of the clutch means that the engine alone cannot be used to drive the propeller. Also in flywheel parallel hybrid systems the motor generator is close coupled (often in a bespoke housing adding to cost) between the engine and gearbox making it difficult to service the motor generator, and the motor generator is subject to the heat of the engine and often needs cooling.
Diesel electric and serial hybrid systems are also well known, and have disadvantages and advantages over parallel hybrid systems.